The number of bits allocated to a pixel on a raster scan display determines the number of different colors that can be simultaneously displayed. For an all-points-addressable (APA) display, there is physical memory allocated to each pixel. A 1024 by 1024 display with eight bits of memory allocated per pixel requires 1 Megabyte of memory for the display buffer, allowing 256 different colors to be simultaneously displayed. It is common to pass the eight-bit pixel value through a lookup table (or palette) before driving the CRT screen. The lookup table typically allows the selection of one of 256 different 24-bit values representing 16 million different colors.
One method of producing the effect of relative color intensity utilizes "dither" patterns, including dots of one color interspersed with dots of another in a correct ratio to form the desired shade. For example, to achieve a mixture of two primaries such as red and yellow, dots of red may be interlaced with dots of yellow, wherein the interlaced arrangement comprises the dither pattern. If there are appreciable numbers of dots or pixels in an area being colored, and if the resulting hard copy is viewed at any distance, the eye combines the colors of the dither pattern to achieve the effect of the desired shade.
The management of the color image including the "rasterizing" of data to supply the individual dots or pixels is usually under the control of a microprocessor. According to the conventional approach, the processor directs the production of each pixel at each particular location on a display device, and the processor looks up the color information in a table in order to determine what color each pixel should be in order to write the overall dither pattern.
In an article entitled "Using Ordered Dithers to Display Continuous Tone Pictures on an AC Plasma Panel", Proc. SID, Fourth Quarter 1974, pp. 161-169; by J. N. Judice et al. A scheme for ordered dither is described. Grey scale images are converted to a single bit per pixel output.
The following patents are extensions of the Judice et al article as they extend the grey scale dither to color dither. Essentially, these patents take a 24-bit color and break it into independent 8-bit red, green and blue components or their complements cyan, magenta and yellow, and sometimes black. These 8-bit images are then converted into 1-bit images via an ordered dither technique as in Judice et al. There is no teaching of converting the 8-bit image into a more than 1-bit image.
U.S. Pat. No. 4,730,185 to Springer et al, sets forth a color display system in which color dither patterns are read into a pixel bit map memory used to form a color display by means of concurrently addressing a pattern memory storing the dither patterns. Lower order address bit repeatedly access a preselected portion of the pattern memory to supply the dither pattern which is written as data into the pixel bit map memory.
U.S. Pat. No. 4,683,466 to Holtey et al, describes a color display graphics system which includes three bit map memories for storing bits representing red, green and blue colors respectively. Combinations of bits from the same address locations of each bit map memory display a pixel which could be any one of eight colors: black, blue, green, cyan, red, magenta, yellow or white. A read only memory (ROM) stores patterns made up of sixteen bits in a four-by-four matrix for each of the red, green and blue colors. The 16-bit matrices are stored in their respective bit map memories for subsequent color display. Combinations of matrices may be used to show shades of the above eight colors and the mixing of any of those shades.
In an article entitled "New Dithering Technique For Realistic Color Image Synthesis on Binary RGB Displays"; Proc. SPIE Int. Soc. Opt. Eng. (USA) Vol. 638 2986 pp. 123-137; by R.J. Gove, a technique similar to those set forth in Judice et al are described. Specifically, a new technique for the displaying of continuous tone color pictures on a bi-level, tri primary display or a display with limited numbers of colors displayable is investigated. The significance of this technique is that it can provide pleasing and realistic images on standard, low cost, RGB graphic display, which is in abundant use in the personal computer community. Specific hardware has been constructed to acquire images from a video camera and generate these images on several personal computer color graphic system monitors. The time to process the digitized 24-plane, RGB image into the dithered 3-plane, RGB image is less than one second.
According to the present invention, dithering is utilized to display a far larger number of colors without increasing the size of the display buffer. This is accomplished through ordered dither and lookup table color selection.